A selective reduction NOx catalyst (hereinafter, SCR) and an occlusion reduction NOx catalyst (hereinafter, LNT), for example, are well know as an exhaust purification catalyst that reduces and purifies nitrogen compounds (hereinafter, NOx) contained in exhaust gas discharged from an internal combustion engine.
In the SCR, the NOx in the exhaust gas can be reduced by promoting the reductive reaction of the NOx with ammonia (NH3) that is supplied as a reducing agent. In the LNT, the NOx in the exhaust gas is occluded when an air-fuel ratio of the exhaust gas is lean and occluded NOx is discharged when the air-fuel ratio is rich by the supplied fuel (HC) such that CO, HC, H2, and so on in the exhaust gas reduces the NOx. For the purpose of supplying the ammonia or the fuel to the exhaust purification catalyst as the reducing agent, a reducing agent injection valve is provided on the upstream side of the exhaust purification catalyst.
Patent document 1, for example, discloses an exhaust gas purification device in which a urea solution injector is provided on the upstream side of the SCR, and which supplies the ammonia to the SCR as the reducing agent by injecting the urea solution from the urea solution injector to the exhaust gas in an exhaust passage.